Lactoferrin (Lf) is the most avid Fe-binding protein in human milk and other epithelial secretions and is the major constituent of the specific granules of neutrophils. Although for some time relegated mainly to a function in Fe absorption and metabolism in the neonate, Lf has recently emerged as a molecule of considerable interest, whose chief role may be as the first line of defense against microbial infections through sequestration of Fe required for microbial growth. Many other diverse activities have been attributed to Lf, from feedback control of myelopoiesis to regulation of NK and macrophage function. Defects in Lf expression are observed in some cancers and could contribute to the pathogenesis or clinical manifestations of the disease. Because of its profound affinity for Fe, and this multitude of Fe related and unrelated effects, Lf offers unusual potential as a novel preventive or therapeutic agent, or as an immunomodulator. However, many of the proposed Lf functions are controversial, and even inconsistent, their mechanistic relationship to Fe-binding is uncertain, and none has been proven operative at any physiologic level. The investigators have undertaken a detailed structure-function analysis of the molecule and the first systematic study of Lf's true in vivo physiologic role. They have made the unique observations that: there are multiple forms of Lf, at least one of which binds no Fe at all and, instead, has ribonuclease activity; Lf possesses potent anti-tumor and anti-metastatic activities in experimental systems which are due to activation of NK cells by a mechanism fully independent of Fe; and, they have demonstrated that the well-known but largely disregarded ability of Lf to bind to DNA occurs with distinct sequence specificity and can lead directly to transcriptional activation. These findings offer a new perspective on Lf, and a new model for its action, one in which Lf no longer serves merely as a passive scavenger of Fe, but also functions as an active and immediate signaling molecule for immune effectors constituting the second wave of defense against infection and, possibly, tumorigenesis. The results also suggest a novel pathway for general cell signaling and regulation -- secreted proteins that are taken up by target cells in which they directly activate gene expression by themselves binding to DNA. Tests of this model, and our continued analysis of the structure and functions of Lf, will be accomplished by: 1. producing and characterizing site-directed mutant forms and fragments of the Lf molecule; 2. identifying the genes activated by Lf using molecular and newly developed genetic approaches; 3. determining whether Lf acts intracellularly in concert with another molecule; and 4. continuing their development of Lf knock-out mice. These studies will help to explain the disparate results obtained with Lf and could provide a rational basis for use of this important and distinctive molecule for nutritional, preventive or therapeutic purposes.